Steam turbines convert the kinetic energy present in a steam flow to rotational energy of a turbine shaft. The steam flow impinges turbine blades connected to the turbine shaft thus causing the shaft to rotate. The turbine shaft is connected to a generator shaft to transfer the rotational energy to the generator for generating electricity according to well-known generator principles.
Typically a steam turbine comprises several turbine stages each optimized for specific steam conditions. As the steam flow passes serially through the turbine stages it gives up kinetic energy to rotate the turbine shaft.
In conventional power plants the steam flow is produced by burning coal or natural gas and using the resulting heat to boil water and create the steam flow. In nuclear power plants fission energy is captured to convert the water to a steam flow.
Geothermal energy is generated and stored in the Earth. The thermal energy (from molten rock and radioactive decay of minerals within the Earth's crust, for example) heats underground water that is trapped within pockets. The water may also rise to the surface through cracks and between geological plates. Wells are drilled into the pockets and the water/steam mixture, or sometimes pure steam, then flows through pipes to the surface. Upon escaping from the high underground pressure, the heated water turns to a mixture of steam and hot water (i.e., a mixed phase fluid). The mixed phase fluid passes through a cyclone separator where the liquid is separated from the steam. The steam is captured and piped to a geothermal turbine. The liquid (also referred to as brine) is re-injected into the earth where it will be reheated. At a few geographic locations, superheated steam is ejected from the earth's crust thus eliminating the need for a separator stage.
The steam strikes turbine blades causing the geothermal turbine shaft to turn. Like the steam turbines described above, the shaft of the geothermal steam turbine is connected to a generator shaft; rotation of the generator shaft generates electricity according to known generator principles.
To further improve operation and efficiency of the geothermal turbine, water and impurities/contaminants (referred to herein as contaminants) are also partially removed from the steam flow prior to entering the turbine. Removing water from the steam flow, i.e., “drying” the steam flow, permits the turbine to operate at a higher efficiency and reliability.
The steam flow rate through the stages of a geothermal turbine (and therefore the generated electricity) varies based on: stage position in the serial string of stages, percent of moisture at the turbine inlet, the number of turbine stages, the exhaust pressure etc.
The advantages of geothermal energy generation include the clean and safe generation process and its use for supplying continuous and reliable base load power for a utility system. Electricity generated from geothermal sources conserves our fossil fuels and contributes to a diverse base load generation along with reduced greenhouse gas emissions.
The temperature of the underground steam may be close to the steam saturation point, and therefore the steam is in near-equilibrium with heated water. If the saturated steam is reduced in temperature or pressure (e.g., as the steam flows through the turbine and relinquishes its kinetic energy to create rotational energy) the steam condenses to produce water droplets.
These condensation droplets can damage blades of the steam turbine. Ideally, the droplets should be removed from the turbine at each turbine stage to prevent impingement on the rotating blades (and resulting corrosion and erosion) and to improve overall performance and efficiency.
Additionally, since the steam originates far below the surface of the earth and flows through layers of rock to reach the earth's surface, it contains various silicates and other contaminants that can both physically and chemically damage the turbine blades, including corrosion-induced fatigue that can lead to premature failure of turbine components, especially the turbine rotor and blades.
To remove the contaminants that have precipitated or coated the turbine internal components, geothermal turbines are periodically water-washed. Water washing involves directing a water stream into the turbine during operation. The turbine stationary nozzles and rotating blades are cleaned by the impingement of the water stream. The water then flows to and collects at the bottom of the turbine casing (enclosure) between each stage.
The condensate (i.e., condensed water) and the washing water collect at the bottom of the turbine casing are removed from the turbine casing through drain pipes.
Prior art geothermal turbines use a water drain system comprising a drain component (e.g., a pipe) at a low point within the casing where the water collects. However the contaminants within the steam/water flow may plug the pipe, thereby preventing effective water removal from the turbine.
With a plugged drain the water remains within the turbine causing erosion and corrosion of its components, especially the turbine blades. Since the pipe is internally fixed to the turbine casing, to access a plugged drain the turbine must be shut down and the casing upper half, the rotor and the turbine diaphragms removed. Only then can one gain access to the plug pipe.
Shutting down the geothermal turbine also results in lost revenue for the owner/operator and material and labor costs expended to diagnosis and conduct the repair.
Also in the prior art it is common for every stage to have the same diameter pipe exiting the turbine casing. This configuration is sub-optimal for both performance and reliability.
In certain prior art turbines each stage is optimized based on steam pressure, temperature, moisture content, differential pressure, the amount of water expected to flow through the pipe, etc. Each stage therefore comprises an appropriately sized (i.e., diameter) drain pipe. Typically each successive drain pipe (following the direction of steam flow and therefore toward the lower pressure stages of the turbine) has a larger diameter. The drain pipes within the water drain system may be sized for each stage in the design phase and then optimized during startup/operation.
But sizing each pipe diameter during the turbine design phase is problematic. To maintain the optimum moisture removal from the turbine casing, the diameter should be as small as permitted by the operating parameters of the turbine stage. In contrast, to avoid pipe clogging by contaminants, the diameter should be as large as possible. Clearly these two interests are in opposition.
Further, the diameter of the pipe may be sized based on an amount of water that is desired to be removed from the incoming steam flow during operation. But components designed to remove this water may not be able to achieve that desired objective. As a result the steam flow impinging the blades may contain more water than expected and desired. This excess water is carried from the turbine by the drain system, which now may be undersized given the additional water volume.